A quasi-phase matched wavelength conversion optical element has been proposed in which a periodic polarization inversion structure is realized by applying a stress to quartz crystal (SiO2), which is a paraelectric material, in the vicinity of the α-β phase transition temperature, so that a periodic twin structure is created (S. Kurimura, R. Batchko, J. Mansell, R. Route, M. Fejer and R. Byer: 1998 Spring Meeting of the Japan Society of Applied Physics Proceedings 28a-SG-18). This is a method in which a quasi-phase matched crystal based on quartz is manufactured by utilizing the twin crystal of quartz to achieve a periodic inversion of the sign of the nonlinear optical constant d11 and d22.
In the case of quartz, the short absorption edge is a wavelength of approximately 150 nm, and ultraviolet absorption at wavelengths shorter than 200 nm is almost neglegible compared to the case of nonlinear optical elements using conventional birefringence phase matching (β-BaB2O4 and CsLiB6O10, etc.) or nonlinear optical elements using the quasi-phase matching of ferroelectric materials (LiNbO3 and LiTaO3, etc.). Accordingly, light with a wavelength of approximately 193 nm comparable to that of an ArF excimer laser can be generated with high efficiency by second harmonic generation, and semiconductor exposure apparatuses using this have also been proposed (Japanese Patent Application Kokai No. 2002-1222898). The crystal axis inversion period in this case is approximately 0.95 μm.
Lithium niobate and lithium tantalate are universally known as conventional quasi-phase matched crystals, and there has been active research aimed at the direct conversion of light, etc., in wavelength-multiplexed optical communications. However, in the case of lithium niobate and lithium tantalate, optical damage caused by the photorefractive effect is a major problem, so that there have been limits to utilization at a high output power. In the case of quartz, on the other hand, there is no optical damage due to the photorefractive effect, so that use in a sufficiently stable state is possible.
Furthermore, quartz itself is a substance with an established production technology; accordingly, quartz can easily be obtained, and costs can be kept low. Furthermore, the mechanical characteristics and chemical properties of quartz are also among the most superior characteristics found in optical crystals. Moreover, hygroscopic nature, which is conspicuously seen in nonlinear optical crystals using conventional birefringence phase matching (for example, β-BaB2O4 and CsLiB6O10 mentioned above), is absent from quartz, so that quartz is extremely advantageous from the standpoint of handling. In addition, the d11 coefficient is approximately 0.3 pm/V, which is slightly smaller than that seen in common nonlinear optical crystals, so that a sufficient conversion efficiency may be expected.
Various methods are known as methods for manufacturing an artificial twin structure in quartz; however, the hot pressing method has currently been proposed as the most influential method (S. Kurimura, I. Shoji, T. Taira, M. Fejer, Y. Uesu and H. Nakajima: 2000 Fall Meeting of the Japan Society of Applied Physics Proceedings 3a-Q-1). In this method, a periodic step structure is formed on the surface of one side of a quartz crystal substrate, this quartz crystal substrate is clamped between heater blocks from above and below, the temperature of the quartz crystal substrate is elevated, and pressure is applied at the point in time at which this temperature reaches a desired temperature. In this case, since stress acts only on the portions corresponding to the protruding parts of the step structure, the crystal axis is inverted only in these portions. These portions with inverted crystal axes grow to the interior of the crystal and are thus propagated into the crystal, so that a periodic twin lattice that penetrates greatly in the direction of depth can be manufactured. Specifically, stress is concentrated only in the protruding portions, and twins are generated from these areas; these twins gradually grow into the interior, so that a twin structure with a large aspect ratio is manufactured.
An example of such a quartz crystal substrate in which a step structure is formed on the surface of one side will be described with reference to FIG. 4. A step structure is formed on the surface of one side of a quartz crystal substrate 1; as a result, protruding parts 2 that have a solid rectangular shape are formed at a specified interval. These protruding parts 2 have a specified width in the left-right direction in the figure, and the plurality of protruding parts 2 are formed at a spacing that is the same as this width in the left-right direction in the figure. The protruding parts 2 have a rectangular solid shape that is long in the direction of depth in the figure. The direction parallel to this longitudinal direction is taken as the direction of the a axis. Naturally, the a axis is perpendicular to the normal of the quartz crystal substrate 1.
The direction of the c axis of the quartz crystal is perpendicular to the a axis; however, this c axis is slightly inclined with respect to the normal direction of the quartz crystal substrate 1, as indicated by S. Kurimura in the May 2000 issue of the Journal of the Japan Society of Applied Physics. Specifically, the quartz crystal substrate 1 is cut so that the normal and the c axis form a slight angle. Twins tend to be generated as this angle increases; in actuality, however, this angle is kept to an angle of approximately 10 to 20 degrees. As is shown in the figure, the light is incident from the end surface of the quartz crystal substrate 1, and the direction of polarization is the same direction as the direction of the a axis. The light whose wavelength is converted inside the quartz crystal substrate 1 is emitted from the end surface that is located on the opposite end from the incident surface.
An example of the method for manufacturing the protruding parts 2 shown in FIG. 4 will be described below. First, a Cr film is formed to a thickness of approximately 100 nm on the surface of the quartz crystal substrate 1 by a sputtering method. The surface of this film is coated with a positive type resist, and the portions other than the portions that will form the protruding parts 2 are exposed and developed using a semiconductor exposure apparatus such as an i-line stepper. Next, the Cr film is removed using the remaining resist as a mask. Then, wet etching is performed by means of hydrofluoric acid using the remaining resist film and Cr film as a mask, so that a step structure with a depth of about a few microns is manufactured. As a result, a quartz crystal substrate 1 is completed which has a step structure with protruding parts 2 such as those shown in FIG. 4 formed on the surface. Furthermore, the Cr film may either be stripped or not stripped prior to pressing.
It is convenient in a wavelength conversion device that the angle between the c axis and the normal of the quartz crystal substrate be as small as possible; however, the stress required for twin formation is increased in this case. Accordingly, the quartz crystal is cut so that the angle between the c axis and the normal of the quartz crystal substrate is an angle ranging from a few degrees to approximately 20 degrees.
Furthermore, although this is not a publicly known technique, the use of dry etching is also conceivable instead of the wet etching mentioned above. In this case, for example, a Cr film is first formed to a thickness of approximately 100 nm on the surface of the quartz crystal substrate 1 by a sputtering method. The surface of this film is coated with a positive type resist, and the portions other than the portions that will form the protruding parts 2 are exposed and developed using a semiconductor exposure apparatus such as an i-line stepper. Then, the Cr film is removed using the remaining resist as a mask. Subsequently, dry etching such as RIE or ICP is performed using the remaining resist film and Cr film as a mask, so that a step structure with a depth of about a few microns is manufactured. Finally, a quartz crystal substrate 1 which has a step structure with protruding parts 2 such as those shown in FIG. 4 formed on the surface is completed by removing the resist film and Cr film.
In order to obtain a quartz crystal substrate which has a twin structure, a pressing apparatus which has a cartridge heater is used, the quartz crystal substrate 1 is clamped from above and below and heated to the vicinity of the phase transition temperature, and pressing is performed when the desired temperature is reached. Consequently, a stress acts only on the protruding parts 2, so that the crystal axis components are inverted only in these parts. Then, the portions in which the crystal axes are inverted grow into the interior of the crystal and propagate into the interior of the crystal, so that a periodic twin lattice that penetrates greatly in the direction of depth can be manufactured. A high aspect ratio in the direction of depth can be created by controlling the pressing time and changes in the pressure over time.
In the above description, a step structure was formed on the surface of the quartz crystal substrate 1; however, it would also be conceivable to polish the surface of the quartz crystal to a planar surface, and to form steps in the pressing surface on the side of the press (this is not a publicly known technique). If a ceramic such as Si3N4 is used as the pressing surface on the side of the press, a periodic step structure can be created using the lithographic technique and working by dry etching described above.
The boundaries of the inversion of the crystal axes can be observed by immersing the quartz crystal substrate that has been pressed as described above in hydrofluoric acid for several minutes, and utilizing the differences in the etching rate. FIG. 5 shows the quartz crystal substrate 1 as seen from the normal direction of the substrate. The areas surrounding the rectangular protruding parts 2 formed on the surface of the quartz crystal substrate 1 are cut away by dry etching or the like. Accordingly, in the case of pressing by the flat surface of the press, the rectangular portions corresponding to the protruding parts 2 should undergo a crystal axis inversion; in actuality, however, triple symmetry about the c axis of the quartz crystal is reflected, so that the crystal axis inversion regions 3 are often produced with a hexagonal pattern as indicated by the hatching in the figure.
Accordingly, in cases where the step period is short, e.g., around a few microns, the crystal axis inversion regions 3 do not spread throughout the entire protruding parts 2 of the steps, and stop at an intermediate point in a hexagonal shape as shown in the figure, so that crystal axis inversion does not occur to the ends of the step pattern (protruding parts 2). In such cases, if laser light which generally has a spread with a radius of several tens of microns or more is caused to be incident, only a very small part of this laser light undergoes wavelength conversion.
Meanwhile, the crystal axis inversion regions of the quartz crystal tend to be produced from the ends of the step pattern (protruding parts 2) where the stress is concentrated. Accordingly, in cases where the step pattern has a long period, e.g., several tens of microns, crystal axis inversion regions are formed only at the ends of the step pattern, and are not formed in the central portions, which is a problem in that the desired twin periodic structure cannot be obtained.
Furthermore, as a result of the concentration of an extremely large stress in the four corners of the protruding parts 2 of the rectangular pattern, a phenomenon may occur in which crystal axis inversion regions are produced in portions outside of the protruding parts 2. When such a phenomenon occurs, especially in cases where the step pattern has a short periodic structure, these portions may be merged with adjacent pattern portions, so that the periodic structure is destroyed.
The present invention was devised in light of such circumstances; the object of the present invention is to provide a method for manufacturing a quasi-phase matched quartz crystal in which crystal axis inversion regions with a desired shape can easily be formed, and a quasi-phase matched quartz crystal which is formed by one example of this method.